First off, I have searched for several days on this question, but none exactly describe the symptoms that my '79 280zx has.

Background: Engine has just been rebuilt, (see my other thread- Worked before...Now doesn't). Compression by far better, but rings not fully seated...Only about 25 miles so far.

The symptoms normally occurs a few minutes after a cold start, (the engine is warming up, but not quite there), and after the engine is warm. They also appear only while under load and I cant reproduce them in the driveway.

The Symptoms: While driving, (1.5k to ~5k RPM), the engine will suddenly loose power, start stumbling and backfiring. edit: This will happen whether it is from a stop or while driving. While it is occuring, if I try to rev the engine, the RPM will not go above about 4000 rpm. If I am still moving I can push in t he clutch, turn off the engine, wait to see the oil pressure gauge go to zero and then restart the engine. Sometimes the symptoms go away, sometimes not.

What I have done/tried:
1- Thinking that the intake was too hot and causing the injected fuel to ignite, I disconnected the cold start valve. No effect
2- Verified the cam/crank/dizzy timing were synced. Correct
3- Verified timing at 10 deg BTC. Correct
4- Checked the ECU connections and conducted every resistance and voltage tests in the '80 Fuel Injection Bible. All readings and measurements within limits
5- New distributor, wires, plugs and tried 3 different coils, (one of them a new MSD coil). Same intermittent symptoms
6- Removed, cleaned in an ultrasonic bath and tested/verified all injectors. All good and produced a decent spray pattern.
7- Removed, disassembled and completely douched the throttle body. Was full of crap. After cleaning, same symptoms.
8- Cleaned and verified AFM and TPS contacts
9- Drained tank and added new gas
10- Verified fuel pressure (in the driveway) at the various manifold pressures as per the EFI Bible. Correct.

What throws me for a loop is the intermittent part. Sometimes it takes me about 30 min to go a couple of miles. Other times this only happens once or twice on the same couple of miles.

If anyone has a suggestions on possible fixes for this I would be truly grateful.

 

I've had a problem with crap in the gas tank. It was plugging my filter and causing it to run lean at high RPM's. I took out the tank and put in muriatic acid and water and sloshed it around for about an hour. Now it works good with a new filter!

 

I checked the fuel pressure from idle up to ~5500 rpm. Pressure was on the high end of the specs. Granted, this was done under no load. This evening I'm going to rig up a pressure meter with a longer hose so that I can test fuel pressure while driving. I am also going to hook up a thermocouple to the fuel rail and see how hot it is getting. I may be getting vapor lock. (This might be a possibility since the symptoms start appearing as and after the engine is warmed up).

Also, if it was fuel related, I would think that would be more constant where as what Im suffering from is more intermittent.

 

Did you check and clean all your grounds and your battery connections are clean and tight? You said you cleaned and verified the AFM contacts, does that include the carbon track? Maybe the Temp sensor is going since it's happening at warm up?

You said you replaced the distributor; was it the distributor itself or just cap, rotor and wires? The old distributor in my Z had some play in it and when you rev'd the motor it would be ok, till you started driving, and right about the point where you describe i would get bucking/sputtering/backfiring. Started at 3000-5000RPM and got worse over a few months till I figured out what it was...

 

audiofreak97: The grounds are clean and so is the battery connections. As far as the distributor goes, it is brand new (reman). Also the cap, rotor, wires and plugs are all new. What do you mean by 'play' in your old distributor. (Rotational play of the shaft/rotor).

I did not open up the side of the AFM and clean in there...Thanks for mentioning that, I didn't even think of that. I will check that out.

Driving it today, it really feels like either the vac advance or the mechanical advance is not working correctly. Where should the vac advance to the distributor come from?

 

man this sounds alot like my 83 was doing, i tried injectors and a few other a while back but now it just sits in the garage cuz i gave up and started driving another one of my cars,,, but mine would sometimes act up cold and clear up when warm then, it flipped and started doing it hot. now it does it no matter what temp, kinda doggy like and it would back fire.

 

Yeah, as it would spin it would wobble, and lose its 'contact' with the cap.

 

Just a little update:

After douching the AFM and TB and re-installing them, I wanted to just verify that the distributor was advancing, so I hooked the vac line from the vapor canister up to the vac advance on the distributor. That reduced the frequency of the symptoms.

A couple of days later on my way to work I noticed the price of gas dropped, so filled up the Z. The entire way to work I kept waiting for the stumbling/hesitation/backfire to occur.

Needless to say I got to work LATE because I felt like taking a detour because I was enjoying no problems.

Returning home from work I took the really long way home just to see if it would act up again. Nope, ran fine.

Guess I am inspecting the fuel pick-up this weekend to see if there are any cracks/holes in the pickup somewhere in the lower half of it.

Thank you all for your suggestions and pointers though.

 

What's the latest? Have you found out the root cause yet? Mine has been good for the last few days. The air temp has dropped and a I did a little "tinkering" with connectors and such. I still have a little of the sputter/stumble now and then but for the most part she runs strong. The dizzy question stands out with me because it seems like I can rotate the rotor with the dizzy installed. Don't think that should happen.

 

Update II:

Went looking for a crack or hole in the pickup tube, found something much more horrifying but easier to fix. The pickup strainer was severally clogged with varnish and crud. 1/2 can of carb cleaner and scrapping the bottom with a screwdriver, the screen was clean.

Next I focused on cleaning the tubing and level sensors. In fact, the whole in-tank assembly.

Pulled the fuel filter in the engine compartment. A little gas dribbled out....Until I gave a little blow on the output side and broke the vacuum. Then a stream of **** came a spewing out. Blowing harder yet, even more came out. Replaced with a new filter. Installed an additional filter between the tank and pump.

Conclusion: My assumptions were way off on what the root cause was. Once this tank of gas is gone, Im going to drop the tank and douche it. Flush out all the hard lines and replace all the fuel hose.

 

Yeah...That shouldn't happen if the oil pump - shaft - distributor are all seated/connected correctly. There will be a little bit of play as you remove the slack in the shaft and gearing, but probably no more than 10 degrees or so.